1. Field of the Invention
The present invention relates to a powder coating composition containing a low temperature curable epoxy resin, and more particularly to a powder coating composition containing a low temperature curable epoxy resin, which can be readily cured at a low temperature, thereby capable of being applied to coating objects that are difficult in elevating temperature, or are thermally sensitive.
2. Description of the Related Art
Epoxy resin is a molecule having two or more epoxy functional groups. Epoxy resin is a kind of thermosetting resin irreversibly crosslinked to form a three-dimensional structure at room temperature or under the influence of heat. However, epoxy resins are different from other thermosetting resins in that curing is not carried out on heating in the absence of a catalyst or a curing agent. An epoxy resin is widely used in molded products, cast products, coatings and the like due to its excellent adhesion, mechanical property, electrical property, chemical resistance, etc.
In particular, the epoxy resin is excellent in adhesion, water resistance, and chemical resistance to various types of coating objects, including metals. Accordingly, the epoxy resin based coatings are widely used instead of alkyd resin or phenolic resin based coatings.
A powder coating composition is a coating film forming composition in the form of powder and does not use a volatile solvent such as an organic solvent or water. The powder coating composition is applied to a coating object and then heat-melted. The powder coating composition has advantages in that it does not use a solvent, can form a thick coating film with just one coating, and can use a high molecular weight resin which is not dissolved in a solvent, compared with a solvent-type coating composition. Accordingly, use and demand for the powder coating composition are increasing in fields of construction materials, automobile parts, pipes, mechanical parts, electronic products, iron furniture and the like.
The powder coating composition is generally produced as follows: a resin for a powder coating composition, a curing agent, a pigment and an additive, and the like are mixed in a blender. The mixture is placed into an extruder or a roll-mill, where it is heated at a temperature in a range of 80 to 130° C., melt-mixed and ground into a powder. Thereafter, a coating object is coated with the ground powder coating composition. Alternatively, the powder coating composition can be produced by mixing the above components in a mixer such as a ball-mill or a ribbon blender at room temperature.
The powder coating composition can be coated by various coating techniques, for example, a spray coating technique, a flow coating technique or an electrostatic coating technique. The electrostatic coating technique is most widely used.
Examples of the resin for the powder coating composition are an epoxy resin and a polyester resin. The epoxy resin is generally a bisphenol-A epoxy resin manufactured by reacting bisphenol-A and epichlorohydrin in the presence of a base or an acid. The bisphenol-A epoxy resin is excellent in chemical resistance and corrosion resistance. However, because it is cured at a high temperature, for thick metallic coating objects that cannot reach an elevated temperature within a short time, a large amount of heat energy must be applied thereto, and thus energy use increases. Further, it is difficult to apply to coating objects that are thermally sensitive.
On the other hand, in glycidyl ethers of conventional bisphenol compounds, there have been a number of suggestions that multifunctional bisphenol epoxy resins obtained by further reacting secondary hydroxyl groups of the glycidyl ethers with epichlorohydrin can be used as sealing materials, laminates of electrical and electronic components, etc (see, U.S. Pat. No. 4,623,701, Japanese Patent Laid-open Nos. Hei 5-5020, 6-248055, 6-298904, etc.). However, these references fail to disclose the manufacture of powder coating using the epoxy resins.